The overall aim of this project is to understand the neural mechanisms of sound localization. These results will help us understand how the brain integrates auditory information from the two ears and produces orienting movements of the head, eyes, and ears to allow close visual and auditory inspection of targets. The experiments are designed to test the relative roles of two circuits that arise in the auditory brainstem to encode the cues necessary to localize sounds and generate the motor programs that orient the head, eyes and ears to the acoustic target. One circuit involves the midbrain nuclei of the inferior and superior colliculi with outputs to motor circuits in the brainstem from the deep layers of the superior colliculus. The other circuit involves projections from the inferior colliculus to the medial geniculate and primary auditory cortex. By inactivating the cortex or the midbrain, the relative roles of these two circuits in orienting behavior will be determined. The second aim will examine the vestibulo-auricular reflex by testing for the reflex following inactivation of the semicircular canals in the vestibular system. We will plug the semicircular canals and hypothesize that this will greatly attenuate the reflex. In animals with mobile pinnae, it is hypothesized that the reflex serves to stabilize the auditory world just like the vestibulo-ocular reflex stabilizes images on the retina in the presence of head movements. The final aim will study the circuitry that moves the pinna as a model motor system and compare it with the well-known oculomotor circuit. We will also examine the effect of immobilizing the pinna on sound localization performance.